1. Field of the Invention
The present invention relates in general to a method of rapidly making inexpensive and good quality fully dense resin or metal moulds which closely approximate the performance of moulds made entirely from metal.
2. Description of Prior Art
In the art of making moulds, important design and manufacturing criteria include high thermal conductivity, high accuracy, high strength, high wear resistance and hence improved tool life. Good quality prior art metal moulds adequately meet these criteria but are costly to manufacture and require long lead times. This fact is an impediment to the development of new products given modern manufacturing realities.
There is an emerging and particular need for rapidly making moulds to accommodate shortened product development cycles as well as to meet a variety of other challenges such as product variants, small production runs and prototyping. Naturally, costs need to be controlled and the quality of the moulds improved where possible and to a greater degree than before. There is accordingly a demand for inexpensive moulds which can be made quickly for use in trials, evaluations or final production.
Various methods and techniques are known for making moulds in a more timely manner. For example, resins are now used to make moulds. The liquid resin is poured into a suitable enclosure containing the pattern to be reproduced and allowed to harden resulting in a mould incorporating the obverse shape of the pattern. The disadvantages of these moulds include poor thermal conductivity, reduced thermal shock resistance, high thermal expansion, high shrinkage during hardening or curing of the resin, low strength and wear resistance, and hence low accuracy and short tool life.
To improve the performance of resin moulds, metal filled resins or polymers have been developed. In order, however, to obtain a significant improvement in the performance of these moulds, a high quantity or proportion of metallic powder in the resin mixture is required. This has the effect of increasing the viscosity of the liquid resin mixture thereby making it difficult to pour and degas.
In order to increase the thermal conductivity and therefore the cooling efficiency of resin moulds, a plurality of metal plates may be used which are placed in the mould enclosure prior to the pouring of the resin so as to be embedded in the body of the mould. The cooling plates may be provided with cooling lines for circulating coolant so as to improve the thermal conductivity of the mould. According to another technique, aluminium honeycomb or metal meshes are embedded in the mould body instead of cooling plates.
These mould making methods using resins are easy to use and adequately address the need for rapid and inexpensive moulds. However, the moulds have disadvantages. The thermal conductivity of resins is poor compared to metals by a factor of about 100 and they accordingly suffer to various degrees from poor thermal conductivity. A disadvantage, for example, of using metal filled resins is that the amount of metallic powders in the mixture is limited by the requirement of reasonable fluidity when pouring the mould. As well, since resins are poor conductors of heat the thermal conductivity of the mould remains inherently low. The relatively large shrinkage of the resin component during curing causes distortion of the mould. Thermal expansion of the mould during operations at elevated temperatures also affects the dimensional stability and life of the mould. Thus, although the incorporation of metal powders in the resin mixture improves the mechanical and thermal properties of these moulds, further improvements remain desirable.
Other prior art examples of rapidly making moulds include Selective Laser Sintering processes which use a laser beam to bond a resin coated metal powder layer by layer to form a mould. Another example is the 3D System Keltool process. In this process a slurry consisting of metal powders and a binder is poured into a silicone mould and allowed to harden. A mould made according to this process requires further heat treatment to remove the binder and to add strength to the mould. An additional disadvantage of these methods is shrinkage which is a significant factor in causing distortion, cracking and reducing accuracy of the mould.
Other methods of making moulds include the use of arrays of elongated elements, such as rods, bars or pins of various cross-sections which are axially adjusted with respect to one another to reproduce the pattern or shape to be moulded on the surface of the array. The recess so formed in the surface of the array becomes the mould when poured. Means to manipulate and axially adjust the rod elements include computer control.
This method is relatively complex to use and suffers from the disadvantage that the mould surface is only an approximation of the pattern. As such, the moulded part requires further machining. Another disadvantage of this type of mould is its reduced strength.
It is therefore an object of the current invention to provide a mould and method of rapidly making a mould having improved qualities such as high thermal conductivity, low shrinkage, high accuracy, high strength, high wear resistance and improved life, which closely approximates the performance of a fully dense metal mould. According to a first aspect of the present invention there is provided a method of making an object having a surface which is an obverse copy of a surface pattern on a second object which comprises placing the second object within a walled enclosure having a closed bottom and open top and orientating the surface pattern on said second object to face upwardly within the enclosure; selecting a plurality of elongated metal rod elements each having a cross section and opposite ends; vertically introducing a sufficient quantity of elongated metal rod elements into said enclosure to fill and pack said enclosure, each said elongated metal element having one end in contact against said surface pattern or bottom, each said elongated metal element being in longitudinal abutment with adjacent elongated metal elements defining spaces therebetween; introducing an infiltration agent into said enclosure over said elongated metal elements and allowing said agent to permeate said spaces; allowing said agent to solidify into an object and separating said object comprising said plurality of elongated metal elements from said pattern and said enclosure.
According to another aspect of the present invention there is provided a method of making an object having a surface which is an obverse copy of a surface pattern of a second object as described above wherein said second object is made of a ceramic material; and wherein said infiltration agent is a molten metal; and wherein said elongated metal elements have a melting point which is higher than that of the metal infiltration agent.
According to yet another aspect of the present invention, there is provided a method of making a copy of an object having a surface pattern comprising placing said object within a walled enclosure having a closed bottom and open top and orientating said surface pattern on said object to face upwardly within the enclosure; pouring a ceramic slurry material into said enclosure and making a ceramic pattern object having a shaped cavity incorporating an obverse copy of the said surface pattern; selecting a plurality of elongated metal elements each having a cross section and opposite ends; vertically introducing a sufficient quantity of elongated metal rod elements to fill and pack said shaped cavity, each said elongated metal element having one end in contact against the surface of said shaped cavity, and each said elongated metal element being in longitudinal abutment with adjacent elongated metal elements defining spaces there between; introducing an infiltration agent into said shaped cavity and allowing said agent to permeate said spaces; and allowing said infiltration agent to solidify into a copy of said object and separating said copy from said ceramic pattern.
According to yet another aspect of the present invention the said infiltration material comprises a metal having a melting point which is lower than the melting point of the elongated metal rod elements and said step introducing said infiltration agent consists of placing the assembly into an oven or furnace to melt the metal.